Centrifuges are widely used for clarifying contaminated liquids, a common example being liquids used for cooling or flushing the “work area” of operational machine tools such as grinders, lathes, milling machines, thread rollers, etc.
Wire Electrical Discharge Machines (Wire EDM(s)) are machine tools in widespread use for precision metal cutting but present a special challenge because they require relatively large volumes of cutting liquids of high clarity while generating large volumes of metal particles which must be carried away by the cutting liquid. More specifically, high clarity de-ionized water is required for Wire EDM machining because this process utilizes a high voltage applied across a small gap to produce arcing between the metal piece being machined and a moving brass wire. The de-ionized water washes away particles (of both the metal piece and the wire) generated in the cutting process. If the de-ionizing is degraded, then the increased conductivity of the water will interfere with the arcing and significantly reduce the speed and accuracy of the cutting process. Advanced Wire EDM machines can generate up to ten pounds of steel particles in one hundred hours of coarse cutting of a steel work piece and an equal amount of brass particles from the moving EDM wire.
Wire EDM also produce particles that are difficult to remove with a centrifuge. “Normal” steel particles have a density of about seven grams per cubic centimeter and are easily removed by a centrifuge where the separating force depends on the density difference between the particles and the carrying liquid. However, the fine particles generated in the Wire EDM cutting process become hydrolyzed and grow in size about seven times. Because of the inclusion of water, the density of these hydrolyzed steel particles is very low, on the order of 1.2 grams per cubic centimeter. Thus, there is a small density difference between these hydrolyzed particles and water, which, of course, has a density of 1.0 grams per cubic centimeter. This small difference in density makes the hydrolyzed particles very difficult to remove with a centrifuge. Studies have shown that effective clarification of Wire EDM liquids requires separation efficiencies 20 to 40 times greater than the efficiencies of known prior art centrifuges. Our invention provides centrifuges that more than satisfy the aforementioned requirement.
Prior art centrifuges have not been satisfactory for providing an adequate supply of clarified, deionized water or other liquid for many applications. For example, while U.S. Pat. No. 3,858,793 teaches a centrifuge with a separation efficiency to yield a satisfactory water clarity for Wire EDM machining, this centrifuge has a major shortcoming because it is not self-cleaning. It uses a replaceable cartridge that must be serviced manually when the cartridge has reached its capacity. Hence, this type of centrifuge is not practical for use to support Wire EDM machining.
At this time, most Wire EDMs utilize integral or associated disposable cartridge filtration systems to provide desired water clarity. None of these systems are completely satisfactory; they are expensive in terms of degrading the productivity of the machine as well as the costs of acquisition and installation of replacement filters and disposal of used filters.
U.S. Pat. No. 3,861,584 teaches a self-cleaning centrifuge but has an unsatisfactory separation of foreign matter from the carrying liquid.